...aaaaaaaaaaaaaaaaaannd it's sorted again!
Turns out that the VGA splitter didn't support 1440x900 (even though it says it's fine with higher resolutions and below). I realised it wasn't a driver issue when I had the same issues with Windows.
i5 4440 3.1Ghz, Asus H87M-E motherboard, 8GB DDR3, 2GB DDR5 nVidia card, 2x500GB Seagate hd, Coolermaster 750W PSU running Mint 17.1 MATE and Win7
HP mini, 1.66 Ghz Intel atom processor, 2 GB DDR3, 320 GB hd, Mint 17 MATE